Support IPv6-only wifi in DiscoveryNetworkMonitor on linux

Using only AF_INET to probe a Wifi interface that IPv6-only will fail,
even though the interface is functional.  This change adds an AF_INET6
check if AF_INET fails.

Bug: None
Change-Id: I81f9c2a2a51041597ee384804018b4de7dccc88d
Reviewed-on: https://chromium-review.googlesource.com/1017344
Commit-Queue: Brandon Tolsch <btolsch@chromium.org>
Reviewed-by: Adam Parker <amp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551796}

chrome/browser/media/router/discovery/discovery_network_list_wifi_linux.cc

@@ -21,8 +21,13 @@ bool MaybeGetWifiSSID(const std::string& if_name, std::string* ssid_out) {
   DCHECK(ssid_out);
 
   base::ScopedFD ioctl_socket(socket(AF_INET, SOCK_DGRAM, 0));
+  if (!ioctl_socket.is_valid()) {
+    // AF_INET is for IPv4, so it may fail for IPv6-only hosts even when there
+    // are interfaces up.
+    ioctl_socket.reset(socket(AF_INET6, SOCK_DGRAM, 0));
     if (!ioctl_socket.is_valid())
       return false;
+  }
   struct iwreq wreq = {};
   strncpy(wreq.ifr_name, if_name.data(), IFNAMSIZ - 1);